The present invention relates to an apparatus for constantly maintaining tension on a magnetic tape in a magnetic recording and reproducing system, such as a video tape recorder, a camcorder, a digital audio tape recorder or the like, and more particularly to a tension servo apparatus having a simple construction for maintaining constant tape tension irrespective of the rotation variation of a supply reel the amount of the amount of tape remaining on the supply reel.
Generally, in magnetic recording and reproducing systems, it is very important to maintain the proper tension on the travelling tape to obtain good product performance. The conventional tape tension servo apparatus combines a band-brake which is wound in the supply reel, a tension arm, a tension pole and a pull spring, to automatically maintain the tension of the tape which runs around a drum.
In FIG. 1, which shows a conventional tape tension servo apparatus, a tape 11 travels toward a drum 14 through a tension pole 13 from a supply reel 12. Although not shown, it is well known that the tape winds around a portion of the drum 14, which has a record/read head therein, and is led via rollers, etc. back to a take up reel.
When the tape tension is above predetermined level, the tape will push tension pole 13 to the right, as shown in the drawing. A tension arm 15, to which tension pole 13 is attached at one end thereof, will rotate clockwise about pin 17 against the force of pull spring 16, increasing the restoring force of spring 16. As a result, a band-brake 18 loosens around the supply reel 12 permitting the supply reel 12 rotate smoothly. As the tape 11 is smoothly released from supply reel 12, the tension on the tape is gradually diminished.
On the other hand, if the tension on the tape 11 is below a predetermined level, the tension arm 15 is rotated counterclockwise due to the force of pull spring 16, causing band-brake 18 to tighten around the supply reel 12 to increase the tape tension.
By the above method, the tension pole 13 is rotated to the left or right according to a amount of tension on the tape, to always maintain the proper tension.
However, the tape tension will vary depending upon the rotation variation of the supply reel, external disturbances, and component clearances. Also, the tape tension will vary depending upon the amount of residual tape wound on the supply reel. In addition, the tension servo control is varied according to the spring constant of the pull spring. Thus, to minimize such variations in the tape tension, it is necessary to provide precision components. For example, the pull spring must be optimally designed, and an electronic actuator and reel motor are used to adjust the tension. However, even so, it is difficult to maintain uniform tape tension, and it is very difficult to provide a compact tape tension servo apparatus.
U.S. Pat. No. 4,133,494 to Mitaka discloses a prior art system for maintaining a uniform tension on a tape. In Mitaka, the system, which includes a known tension servo apparatus using a spring, also uses a circuit for detecting the speed of a servo motor or a guide roller and for adjusting the speed of the servo motor and a reel motor.